Oral polio vaccine, given here to a boy in Uttar Pradesh, India, could be combined with injected vaccinations to help eliminate polio globally. |Rotary International in Great Britain and Ireland/ Flickr

Giving the inactivated poliovirus vaccine (IPV) to individuals who have received the live, orally administered vaccine (OPV) boosts immunity against the disease, a new study suggests. The results, published in the 22 August issue of the journal Science, may help resolve controversy over vaccine choice as researchers work to eliminate final poliovirus reservoirs in places like Syria and Iraq.

The use of IPV reduced how much infectious poliovirus children shed in their stools — up to 74% in some cases — and also quadrupled the levels of polio antibodies in many of the children, the researchers found.

"The global eradication effort is at a critical crossroad," said Hamid Jafari, the World Health Organization's director for polio operations and research, and the lead author of the report. "Endemic polio is increasingly geographically restricted to populations in insecure and inaccessible areas. Yet the virus in these areas persists with incredible tenacity."

The two types of polio vaccination work differently within the body to protect against the disease. | Science/AAAS

Two types of vaccine protect against polio, which can cause lifelong paralysis. IPV, which uses killed poliovirus strains, is administered via injection, while OPV, which uses live but weakened strains, is administered orally.

Since the development of the polio vaccine, global efforts to eradicate the virus relied mainly on OPV; for example, it was the vaccine of choice of the Global Polio Eradication Initiative, a public-private partnership spearheaded by WHO in 1988.

"OPV has a superior ability to induce mucosal immunity, is easy to administer requiring no needles, and is substantially cheaper," said Caroline Ash, a senior editor at Science. Mucosal immunity, the protection offered by mucosal linings like the gut and lungs, is needed to stop the person-to-person spread of the virus.

However, despite OPV's successes the choice between OPV and IPV in places where polio lingers continues to be widely debated, in large part because mucosal immunity diminishes rapidly after OPV treatment. Thus, health providers must give several doses of the OPV vaccine, a challenge in remote areas or conflict zones like Afghanistan, Nigeria, and Pakistan.

Furthermore, individuals vaccinated with OPV may still shed polio virus in their stools, which creates risk of the virus spreading to anyone who comes in contact with the infected fecal matter.

"Thanks to OPV, polio has been reduced from more than 350,000 cases every year in more than 125 endemic countries to fewer than 500 cases last year and three endemic countries," said co-author Roland Sutter, the WHO coordinator for research and product development, polio operations, and research. "But what we are after now is eradicating the remaining 1% of cases, from the most challenging areas of the world."

Jafari and colleagues tested whether use of both vaccines, instead of OPV alone, would reduce viral shedding and improve mucosal immunity. Although administering IPV after OPV is known to close certain immunity gaps, its specific effect on intestinal mucosal immunity is less well-known.

The team conducted a randomized clinical trial in nearly 1,000 infants and children in Uttar Pradesh, a state in northern India. The state's dense population, inadequate sanitation, and high incidence of diarrheal disease, among other factors, have made it difficult to eradicate polio there, Sutter said. "Whatever results we would find here in this setting, we knew we would be able to apply everywhere else as well."

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The children were given either IPV or OPV. After four weeks, every child, regardless of the initial dose type, got a dose of OPV. The team then collected and tested more than 3,000 blood samples from the children, and more than 5,000 stool specimens.

In children who were given IPV first, viral shedding in stools was reduced, meaning the children were much less infectious to other children. Jafari and colleagues also found, based on studying blood samples, that the single dose of IPV boosted intestinal mucosal immunity more effectively in children than did an additional OPV dose.

In a 20 August teleconference, WHO researcher and co-author Bruce Aylward replied to a reporter who asked about the cost of IPV. "Until a few years ago, the best price we could get for IPV was nearly $3 a dose...but as a result of some negotiation, UNICEF has secured a price of $1 a dose for some countries." Aylward noted that this is still far more expensive than OPV, which comes in at about 15 or 16 cents a dose.

"The answer is now very clear: Both vaccines complement one another," Sutter concluded."To attain a polio-free world in the most rapid and effective way possible, the vaccines should be used together to interrupt the final chains of virus transmission."